Re: How to weigh a fish without the interference of percentage error of water?

I agree that weighing the tank with and without guppies to subtract for the 
mass of the guppies will not work.  The water weighs too much and the 
guppies too little.  If you have access to a scale that can go down below 
1g in measurement, I suggest taking individual or known numbers of guppies 
out of the eater and weighing them on a piece of paper directly on the 
scale.  They can survive out of water for a minute or two.  The problem 
will be to keep them from flipping off of the scale, but a simple cover 
should do the trick.  If the weight of the guppies is still too small, I 
suggest weighing larger number of guppies and dividing by the total number 
to determine the weight/guppy.  Good luck!  

John F. Rawls, Washington University